Underpinning of existing foundations - what is required?

When converting or refurbishing existing buildings, underpinning may be required to stabilise and strengthen the foundations.

But what foundation issues can be rectified using underpinning and what is involved in the process?

What can underpinning be used for?

Stabilising foundations undergoing settlement.

Stabilising foundations that have moved as a result of landslip or unstable slopes.

Stabilising foundations supported on unconsolidated made ground.

Stabilising foundations affected by erosion.

Stabilising foundations affected by subsidence or heave.

Stabilising foundations affected by mine workings.

Stabilising foundations affected by vibration damage.

Strengthening foundations subject to an increase in load or alterations to existing load path.

To accommodate a new adjoining building structure, a new basement storey or deep sewer.

How to identify when underpinning should be used?

It is important to hire a structural engineer to conduct an appraisal of the existing building and its foundations in order to identify the cause of the issue before starting the underpinning process.

Where the existing building shows evidence of structural damage, the engineer should include a number of principal objectives in the appraisal.

Here’s what to look out for…

A preliminary assessment as to whether the damage is significant.

Cause of the damage - a programme of monitoring is commonly required and can take many months to yield valuable information.

Can the movement be immediately resolved to achieve stability?

Further investigation to confirm size and depth of existing foundations and/or determine the nature and condition of underlying soils. Usually carried out by a specialist ground investigation company, sometimes with assistance from a ground works contractor.

Where partial underpinning is proposed, the expert or underpinning contractor should ensure there is no risk of differential movement between treated and untreated areas.

If the underpinning works are to be carried out adjacent to or in close proximity to an existing building, the client will need to comply with the requirements of the Party Wall Act 1996 and enter into a party wall agreement with the adjoining owner.

Where underpinning is required, structural repairs or strengthening work to the superstructure may also be required.

What is involved in underpinning?

The underpinning of foundations involves increasing the depth of existing foundations in order to transfer the building load to a more supportive soil type.

Although there are a number of different techniques available for underpinning foundations the most common method is traditional mass concrete underpinning.

This is a simple technique that involves excavating a segment of ground below the existing building foundation in controlled stages, to a depth where suitable soil conditions exist. The excavation is then filled with concrete and allowed to cure before the next ‘pin’ is excavated as shown in the diagram below.